Merge configurations with multiple layout files

Add a test for multiple tenants with a partially shared config.

We might use something like this in OpenStack to define common
pipelines but then have separate tenant config files for groups
of projects.

Change-Id: I29dc9327e3d72d5f6797eb2c366c36fe5be8ea8e
diff --git a/tests/fixtures/config/multi-tenant/common.yaml b/tests/fixtures/config/multi-tenant/common.yaml
new file mode 100644
index 0000000..d36448e
--- /dev/null
+++ b/tests/fixtures/config/multi-tenant/common.yaml
@@ -0,0 +1,14 @@
+pipelines:
+  - name: check
+    manager: IndependentPipelineManager
+    source:
+      gerrit
+    trigger:
+      gerrit:
+        - event: patchset-created
+    success:
+      gerrit:
+        verified: 1
+    failure:
+      gerrit:
+        verified: -1